With a message of economic populism, presidential contenders from both parties are fighting for votes from the middle class.

"Far too many Americans live on the edge of economic ruin. They see only a small portion of the population riding the economy's 'up' escalator," noted Jeb Bush. In her campaign announcement, Hillary Clinton said, "the deck is still stacked in favor of those at the top. Everyday Americans need a champion."

These are lofty sentiments. But unless a candidate supports the sorts of jobs that are key to middle-class livelihoods, such words are simply rhetoric. That's why every candidate needs to support infrastructure jobs in the energy sector.

Consider President Obama's recent rejection of Keystone XL. The proposed 1,700-mile pipeline would have transported crude oil from western Canada to the Gulf of Mexico.

According to the U.S. State Department, Keystone XL would have created 42,000 jobs. It would have also produced an additional 20,000 manufacturing positions, plus 118,000 spin-off jobs generated by the supply chain necessary to complete the pipeline.

The federal government studied Keystone XL for seven years for every conceivable pitfall, and consistently found the pipeline to be environmentally sound. But that didn't stop green radicals with no regard for middle-class jobs from blocking its construction. And it certainly didn't stop the president from catering to these environmentalists' agenda.

But the endless delay and recent rejection of Keystone XL is just the best-known casualty of this fanaticism. Numerous other job-creating pipeline projects are also languishing because activists have prioritized their rhetoric over America's economic well-being.

For example, the Sandpiper pipeline would move oil from North Dakota to Minnesota and Wisconsin and create 1,500 jobs. The Constitution pipeline would transport gas between New York and Pennsylvania, creating 1,300 jobs.

Yet both have been delayed and continue to face opposition despite being cleared by state and federal regulatory agencies.

Opponents of infrastructure jobs often argue that such positions are "temporary" and somehow insignificant. Apart from the strange notion that we should turn our noses up at any good-paying job, this line of reasoning shows a fundamental misunderstanding, likely the conceit of those who have never made a living using their hands.

Throughout their working lives, construction professionals work on numerous jobs in numerous locations. There are no "permanent" construction projects. Workers finish one job and then move on to the next. Along the way, they cultivate skills and build a career.

Pipeline projects also provide a fertile training ground for new workers. Through apprenticeship programs, unions use such projects to put people on the road to the middle class.

Pro-energy policies are pro-prosperity policies. Indeed, a new study by Wood McKenzie found that energy-friendly policies would produce 2.3 million new jobs by 2035. If candidates are serious about helping the middle class, they must make supporting energy infrastructure projects a central part of their plans.